Environment and Mission
The environment in which the aircraft is expected to operate plays a very important role in the concept design process. For example, an aircraft designed to carry a certain payload over a certain range may look significantly different (in terms of its wing and powerplant, which are the main quantitative considerations in this early phase of the process) if it is to fly out of “mile high” Denver in August from what it would look like if it was to shuttle between two sea-level research stations in Antarctica.
Incidentally, playing with these numbers as part of the concept phase of the design could reveal the importance (or otherwise) of bespoke airframes. In other words, it may help answer questions like: ‘do we need to provide different wings for different applications or do the added development and life-cycle costs outweigh the performance gains that can be achieved through customization?’
The physics of the atmospheric environment in which aircraft operate is complicated and, in most cases, significantly variable with season, weather, and climate. However, in keeping with the general fidelity level of conceptual studies, it is generally considered acceptable to use the simple model of the International Standard Atmosphere (ISA) as an acceptable approximation. The ISA is generally very close to the true “mean” atmosphere at mid-latitudes and anywhere on the globe at low altitudes.
ISA conditions generally constitute a good baseline, but once a feasible design is found, it may be good practice to revisit the design calculations discussed in the latter half of this chapter and try a range of, say, take-off altitudes and temperatures representative of the operating conditions of the aircraft and check that the chosen design still meets the performance requirements.